In the 0.5-6 THz (1012 Hz) frequency regime, imaging and spectrometric systems have important applications in security, health, remote sensing and basic sciences. Tera-Hertz waves are strongly attenuated by water, but have otherwise a large penetration-depth without causing damage to biological tissues. They are therefore particularly suitable for security applications involving low-risk imaging through opaque objects such as clothes, teeth, paper, plastic and ceramic materials. THz waves are also ideal in health applications such as early skin cancer diagnostics. Thus, extensive research lately has identified many society-essential applications in security, medicine, bioanalysis, remote sensing for environmental monitoring and natural disaster mitigation. With their high frequencies, THz waves are also well suited for extreme wideband communications.
The THz spectral region, however, has so far resisted most attempts to harness its potential for everyday applications. This has led to the expression ‘the THz gap’, which loosely describes the lack of adequate technologies to effectively bridge the region between microwave frequencies below 1 THz and optical frequencies above 6 THz, and more particularly, the lack of a practical source with a useful power level in this particular frequency range. Today, semiconductor electronics and laser optics work from opposite directions to narrow the THz gap. Advanced semiconductor technologies including silicon-CMOS, SiGe HBT and compound-semiconductor HEMTs (high electron mobility transistors) have greatly advanced the art in millimeter wave technology. Yet, the projected attainable frequency by the most robust and cost-effective SiGe HBT technology is currently about 0.5 THz. In the optics region, modern solid-state lasers that rely on transitions from well-defined electronic states encounter a severe challenge in breaking the 6-THz barrier, since such frequency is equivalent to kT=26 meV, the energy of thermal fluctuations at room temperature.
Presently, it is possible to enter the THz gap by means of passive devices such as frequency multipliers. However, such devices generally suffer from significant power losses, which lead to a power-to-system volume that will be impractically small when it comes to the applications. Small and efficient active THz devices are therefore the only solution. Vacuum electronic devices, including klystrons, have been considered as one way to bridge the THz gap. Such devices can perhaps find military and aerospace interest, but their large size, aggressive energy appetite and poor reliability are foreseen to prevent them from penetrating into the vast field of civilian applications for security and health. Solid-state electronics based on advanced semiconductors is therefore seen as the only possibility, especially for battery-powered portable THz systems for use in our everyday life.
CMOS-based solutions for 1 THz operation require transistors with a 10-nm channel length. At this gate length, however, very low output power is expected due to quantum tunneling. With superior transconductance and noise properties, SiGe HBT technology is generally considered to provide the most robust and cost-effective solutions for the emerging high-frequency markets. Currently, the enabling technology for SiGe HBT is chemical vapour deposited (CVD) SiGe. The most advanced SiGe HBT today has a maximum transit frequency of 0.4 THz at room temperature. An ongoing European FP7 effort named “DOTFIVE” comprising leading European semiconductor enterprises aims to bring 0.5 THz SiGe HBT technology to the market in 2013. Noteworthy in the DOTFIVE project is the current progress in circuit design of complete frequency multiplier chains for 0.325 THz, which is not only a very lossy method, but also fails to get into the TaraHertz gap.